1. Field of the Invention
The present invention relates to a color management method for the conversion of color in a source device color space to color values in a device space that includes a black channel, such as CMYK.
2. Description of the Related Art
Typically, color management systems utilize color transforms to convert colors from device-dependent to device-independent color spaces, and vice versa. This is beneficial because different devices display colors in different ways. For example, display devices often display colors as a mixture of red, green, and blue light (RGB), while printers will often utilize mixtures of cyan, magenta, yellow and black inks (CMYK) to print colors. It is beneficial to maintain color fidelity across these two different devices, so that an image displayed on a RGB display appears the same when it is printed on a CMYK printer. To maintain this color fidelity, the RGB values are converted into a device-independent color appearance space, and then the colors in the device-independent color space are converted to CMYK values. In this way, the colors can be accurately mapped and reproduced between different devices.
This mapping is typically achieved with color transforms. Conventionally, color transforms are created by measuring the output of a specific device at a plurality of known inputs. For example, a color transform for a CMYK printer can be computed by measuring color patches printed by the printer at a plurality of CMYK values. The measurements are taken by a color measuring device, such as a calorimeter or a spectrophotometer, and the measurements are in a device-independent color space, such as CIEXYZ, CIELab, CIECAM, or CIELuv. With these measurements, a color management system is able to predict what device-independent color a printer will produce at a specific value of CMYK.
However, color management systems ordinarily rely on a conversion in the reverse direction, from a three-dimensional device-independent color value to a four-dimensional CMYK device-dependent value, and this conversion is complicated by the additional degree of freedom provided in the black (K) channel. As with other color spaces having four or more color components, the CMYK space is underspecified. In other words, the same color can theoretically be produced using different amounts of black ink (K) with varying amounts of cyan, magenta, and yellow (CMY). Therefore, a one-to-one unique mapping from color appearance space to CMYK space is not mathematically possible, nor is even desirable since this underspecification provides flexibility in selecting ink values. However, this flexibility can lead to potentially unsatisfactory conversions, since certain levels of black ink may produce visually unsatisfactory colors. For instance, neutral colors, such as grays, typically look better with higher amounts of black ink, even though mathematically accurate conversions can be found with low amounts of black ink. On the other hand, saturated non-neutral colors appear sharper with lower amounts of black ink. As such, it may be desirable to reproduce the black ink characteristics of a source device in a CMYK device so that colors produced by the two devices appear more similar.
In addition, it may be desirable to control the amount of black ink for other reasons. For instance, it may be desirable to always use the most amount of black ink possible, as black ink is generally the least expensive. Alternatively, it may be beneficial to use less black ink and more colored ink, as this may improve the results in a color trapping process.
Attempts have been made to control the K level in conversions to CMYK device space, such as conversions from RGB device space or from another CMYK device space. However, these attempts have not satisfactorily addressed the problems associated with controlling the K level in a conversion from color appearance space to CMYK space. Other attempts have been made to treat the conversion of neutral and non-neutral color appearance space values differently, but have generally been unsatisfactory. Moreover, all such prior attempts do not adequately address the possibility that the source device space might be a device space (such as RGB) that does not include a black channel, or might be a device space (such as CMYK) that does.
U.S. patent application Ser. No. 10/841,184, filed on May 6, 2004, and U.S. patent application Ser. No. 10/953,705, filed on Sep. 28, 2004 describe methods for controlling the K value in conversions from internal color space to CMYK. While the methods described in these applications perform well for controlling the K level in such conversions, it would be beneficial to obtain a more accurate calculation of the amount of black ink used by a destination CMYK device while flexibly accommodating the possibility that the source device might or might not have a black channel.